1. Field of the Invention
The present invention relates to a resistor material, a resistive paste and a resistor using the resistor material, and a multi-layered ceramic substrate. In particular, the present invention relates to a resistive paste capable of being baked in a neutral or reducing atmosphere, a resistor material advantageously included in this resistive paste and a resistor obtainable by using this resistive paste.
2. Description of the Related Art
A circuit pattern constituted of electrodes and resistors has been usually formed on a ceramic substrate comprising alumina so that various kinds of electronic components can be mounted thereon. A metallic paste of precious metals such as silver and a silver-palladium alloy is conventionally screen-printed and baked in the air in order to form electrodes or electrode patterns.
A high-density printed circuit is desirably formed by laminating the substrates in order to form a three dimensional wiring circuit, thereby attempting to further compact the electronic appliances. However, high melting point metals such as tungsten and molybdenum should be used as conductive materials in forming wiring layers in the laminated substrate when conventional general purpose alumina substrates are used because the sintering temperature of alumina is high. Therefore, the application field of the printed circuit is restricted since the metals described above have high resistivity.
For the purpose of solving the problems as hitherto described, a low sintering temperature substrate (for example, a composite substrate comprising a ceramic and glass) that can be sintered at a low temperature (1000.degree. C. or less) and that is able to use such metals as silver, palladium or copper as metallic inner layer materials have been used in recent years.
The precious metal paste as described above may be used for the electrode materials in such low sintering temperature substrate. However, the precious metal paste is not only expensive but also has a high impedance and has a drawback in that the practical problem electromigration is liable to occur.
On the contrary, it was found and noticed that a high quality electrode pattern could be produced with a cheap production cost when a paste containing a base metal such as copper that has low impedance and does not cause electromigration is screen-printed on a sintered substrate or on a green sheet to be sintered in a neutral or reducing atmosphere.
The resistor provided on the substrate so as to put a plurality of base metal electrodes formed after baking the base metal paste into electrical continuity, or the resistive paste for forming the resistor pattern can not be used when RuO.sub.2 based materials are used as conductive materials since they are liable to be reduced. A material capable of being baked in a neutral or reducing atmosphere such as nitrogen is desirable.
In compliance with the foregoing requirements, a LaB.sub.6 based resistive paste disclosed in Japanese Examined Patent Publication No. 55-30889, a NbB.sub.2 based resistive paste disclosed in Japanese Unexamined Patent Publication No. 63-224301 and a resistive paste of Nb.sub.x La.sub.1-x B.sub.6-4x solid solutions disclosed in Japanese Unexamined Patent Publication No. 2-249203 are proposed as the resistive paste capable of being baked in a neutral or reducing atmosphere.
A resistor formed by using Nb.sub.x La.sub.1-x B.sub.6-4x as conductive materials has an advantage over the resistor formed by using resistive pastes such as LaB.sub.6 in that the former has a wide range of surface resistivity with good repeatability when the mixing ratio between the conductive material and glass frit is adjusted.
However, the temperature coefficient of resistance (referred to TCR hereinafter) of the resistor formed of Nb.sub.x La.sub.1-x B.sub.6-4x based resistive pastes tends to shift toward positive (+) direction and its absolute value leaves from zero in the low surface resistivity region (about 10.OMEGA./.quadrature. to 1 k.OMEGA./.quadrature.). Although it was disclosed in Japanese Unexamined Patent Publication No. 7-192903 that TCR on the alumina substrate can be shifted toward the minus direction by adding TiO.sub.2 as a first additive and Co.sub.3 O.sub.4, CoO and Fe.sub.2 O.sub.3 as second additives, the TCR regulating effect and repeatability are not sufficient especially in the low resistance region when the resistor is formed on a low sintering temperature substrate such as a composite substrate of a ceramic and glass, and it is currently impossible to obtain the required TCR characteristic.
A similar situation is found in the resistor formed of a resistive paste such as NbB.sub.2.
On the other hand, the TCR of the resistor formed of Nb.sub.x La.sub.1-x B.sub.6-4x based resistive pastes tends to shift toward negative (-) direction and its absolute value leaves from zero in the high surface resistivity region (about 10k.OMEGA./.quadrature. or more). Although it was disclosed in Japanese Unexamined Patent Publication No. 5-335107 that TCR on the alumina substrate can be shifted toward the plus direction by adding any one or a plurality of B.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3, CrB, NiB, TaSi.sub.2, Ta and AlN, the TCR regulating effect and repeatability are not sufficient especially in the low resistance region when the resistor is formed on a low sintering temperature substrate such as a composite substrate of a ceramic and glass, and it is currently impossible to obtain the required TCR characteristic.